PTRACE(2) NetBSD System Calls Manual PTRACE(2)
NAMEptrace -- process tracing and debugging
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <sys/types.h>#include <sys/ptrace.h>intptrace(int request, pid_t pid, void *addr, int data);
DESCRIPTIONptrace() provides tracing and debugging facilities. It allows one
process (the tracing process) to control another (the traced process).
Most of the time, the traced process runs normally, but when it receives
a signal (see sigaction(2)), it stops. The tracing process is expected
to notice this via wait(2) or the delivery of a SIGCHLD signal, examine
the state of the stopped process, and cause it to terminate or continue
as appropriate. ptrace() is the mechanism by which all this happens.
The request argument specifies what operation is being performed; the
meaning of the rest of the arguments depends on the operation, but except
for one special case noted below, all ptrace() calls are made by the
tracing process, and the pid argument specifies the process ID of the
traced process. request can be:
PT_TRACE_ME This request is the only one used by the traced process; it
declares that the process expects to be traced by its par-
ent. All the other arguments are ignored. (If the parent
process does not expect to trace the child, it will proba-
bly be rather confused by the results; once the traced
process stops, it cannot be made to continue except via
ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as
execv(3)), it will stop before executing the first instruc-
tion of the new image. Also, any setuid or setgid bits on
the executable being executed will be ignored.
PT_READ_I, PT_READ_D
These requests read a single int of data from the traced
process' address space. Traditionally, ptrace() has
allowed for machines with distinct address spaces for
instruction and data, which is why there are two requests:
conceptually, PT_READ_I reads from the instruction space
and PT_READ_D reads from the data space. In the current
NetBSD implementation, these two requests are completely
identical. The addr argument specifies the address (in the
traced process' virtual address space) at which the read is
to be done. This address does not have to meet any align-
ment constraints. The value read is returned as the return
value from ptrace().
PT_WRITE_I, PT_WRITE_D
These requests parallel PT_READ_I and PT_READ_D, except
that they write rather than read. The data argument sup-
plies the value to be written.
PT_CONTINUE The traced process continues execution. addr is an address
specifying the place where execution is to be resumed (a
new value for the program counter), or (caddr_t)1 to indi-
cate that execution is to pick up where it left off. data
provides a signal number to be delivered to the traced
process as it resumes execution, or 0 if no signal is to be
sent. If a negative value is supplied, that is the nega-
tive of the LWP ID of the thread to be resumed, and only
that thread executes.
PT_KILL The traced process terminates, as if PT_CONTINUE had been
used with SIGKILL given as the signal to be delivered.
PT_ATTACH This request allows a process to gain control of an other-
wise unrelated process and begin tracing it. It does not
need any cooperation from the to-be-traced process. In
this case, pid specifies the process ID of the to-be-traced
process, and the other two arguments are ignored. This
request requires that the target process must have the same
real UID as the tracing process, and that it must not be
executing a setuid or setgid executable. (If the tracing
process is running as root, these restrictions do not
apply.) The tracing process will see the newly-traced
process stop and may then control it as if it had been
traced all along.
Three other restrictions apply to all tracing processes,
even those running as root. First, no process may trace a
system process. Second, no process may trace the process
running init(8). Third, if a process has its root direc-
tory set with chroot(2), it may not trace another process
unless that process's root directory is at or below the
tracing process's root.
PT_DETACH This request is like PT_CONTINUE, except that after it suc-
ceeds, the traced process is no longer traced and continues
execution normally.
PT_IO This request is a more general interface that can be used
instead of PT_READ_D, PT_WRITE_D, PT_READ_I, and
PT_WRITE_I. The I/O request is encoded in a ``struct
ptrace_io_desc'' defined as:
struct ptrace_io_desc {
int piod_op;
void *piod_offs;
void *piod_addr;
size_t piod_len;
};
where piod_offs is the offset within the traced process
where the I/O operation should take place, piod_addr is the
buffer in the tracing process, and piod_len is the length
of the I/O request. The piod_op field specifies which type
of I/O operation to perform. Possible values are:
PIOD_READ_D
PIOD_WRITE_D
PIOD_READ_I
PIOD_WRITE_I
See the description of PT_READ_I for the difference between
I and D spaces. A pointer to the I/O descriptor is passed
in the addr argument to ptrace(). On return, the piod_len
field in the I/O descriptor will be updated with the actual
number of bytes transferred. If the requested I/O could
not be successfully performed, ptrace() will return -1 and
set errno.
PT_DUMPCORE Makes the process specified in the pid pid generate a core
dump. The addr argument should contain the name of the
core file to be generated and the data argument should con-
tain the length of the core filename. This ptrace call
currently does not stop the child process so it can gener-
ate inconsistent data.
PT_LWPINFO Returns information about a thread from the list of threads
for the process specified in the pid argument. The addr
argument should contain a ``struct ptrace_lwpinfo'' defined
as:
struct ptrace_lwpinfo {
lwpid_t pl_lwpid;
int pl_event;
};
where pl_lwpid contains a thread LWP ID. Information is
returned for the thread following the one with the speci-
fied ID in the process thread list, or for the first thread
if pl_lwpid is 0. Upon return pl_lwpid contains the LWP ID
of the thread that was found, or 0 if there is no thread
after the one whose LWP ID was supplied in the call.
pl_event contains the event that stopped the thread. Pos-
sible values are:
PL_EVENT_NONE
PL_EVENT_SIGNAL
The data argument should contain ``sizeof(struct
ptrace_lwpinfo)''.
PT_SYSCALL Stops a process before and after executing each system
call.
PT_SYSCALLEMU
Intercept and ignore a system call before it has been exe-
cuted, for use with PT_SYSCALL.
Additionally, the following requests exist but are not available on all
machine architectures. The file <machine/ptrace.h> lists which requests
exist on a given machine.
PT_STEP Execution continues as in request PT_CONTINUE; however as
soon as possible after execution of at least one instruc-
tion, execution stops again. If the data argument is
greater than 0, it contains the LWP ID of the thread to be
stepped, and any other threads are continued. If the data
argument is less than zero, it contains the negative of the
LWP ID of the thread to be stepped, and only that thread
executes.
PT_GETREGS This request reads the traced process' machine registers
into the ``struct reg'' (defined in <machine/reg.h>)
pointed to by addr. The data argument contains the LWP ID
of the thread whose registers are to be read. If zero is
supplied, the first thread of the process is read.
PT_SETREGS This request is the converse of PT_GETREGS; it loads the
traced process' machine registers from the ``struct reg''
(defined in <machine/reg.h>) pointed to by addr. The data
argument contains the LWP ID of the thread whose registers
are to be written. If zero is supplied, the first thread
of the process is written.
PT_GETFPREGS This request reads the traced process' floating-point reg-
isters into the ``struct fpreg'' (defined in
<machine/reg.h>) pointed to by addr. The data argument
contains the LWP ID of the thread whose registers are to be
read. If zero is supplied, the first thread of the process
is read.
PT_SETFPREGS This request is the converse of PT_GETFPREGS; it loads the
traced process' floating-point registers from the ``struct
fpreg'' (defined in <machine/reg.h>) pointed to by addr.
The data argument contains the LWP ID of the thread whose
registers are to be written. If zero is supplied, the
first thread of the process is written.
PT_DUMPCORE Cause the traced process to dump core. If the addr argu-
ment is not NULL it is taken to be the pathname of the core
file to be generated and the data argument should contain
the length of the pathname. The pathname may contain %
patterns that are expanded as described in sysctl(8). If
the data argument is NULL, the default core file path gen-
eration rules are followed.
ERRORS
Some requests can cause ptrace() to return -1 as a non-error value; to
disambiguate, errno can be set to 0 before the call and checked after-
wards. The possible errors are:
[EAGAIN] Process is currently exec'ing and cannot be traced.
[EBUSY]
· PT_ATTACH was attempted on a process that was already being
traced.
· A request attempted to manipulate a process that was being
traced by some process other than the one making the
request.
· A request (other than PT_ATTACH) specified a process that
wasn't stopped.
[EINVAL]
· A process attempted to use PT_ATTACH on itself.
· The request was not a legal request on this machine archi-
tecture.
· The signal number (in data) to PT_CONTINUE was neither 0
nor a legal signal number.
· PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was
attempted on a process with no valid register set. (This
is normally true only of system processes.)
[EPERM]
· A request (other than PT_ATTACH) attempted to manipulate a
process that wasn't being traced at all.
· An attempt was made to use PT_ATTACH on a process in viola-
tion of the requirements listed under PT_ATTACH above.
[ESRCH] No process having the specified process ID exists.
SEE ALSOsigaction(2), signal(7)BUGS
On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and
similar calls, but the NPC is set willy-nilly to 4 greater than the PC
value. Using PT_GETREGS and PT_SETREGS to modify the PC, passing
(caddr_t)1 to ptrace(), should be able to sidestep this.
NetBSD 6.0 August 31, 2011 NetBSD 6.0

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